Resolving Hydraulic Resistances in Thin-Film Composite Polyamide Nanofiltration Membranes

Thin-film composite (TFC) nanofiltration (NF) membranes are widely used in water treatment and resource recovery. Researchers generally take for granted that the dense polyamide rejection film dictates the overall hydraulic resistance of these membranes, neglecting the contributions of the substrate and the transverse transport of water to reach substrate pores. To address this critical gap, we developed a resistance-in-series model to quantify the resistances from the polyamide film, substrate, and transverse transport. Calibration with multiple experimental data sets revealed that the polyamide film resistance varied over a wide range of 2.90 × 10¹² to 40.15 × 10¹² m^-1, strongly correlating to film thicknesses (correlation coefficients >0.95), with a thickness-normalized resistance of (0.44 ± 0.12) × 10¹² m^-1 nm^-1. Contrarily, the intrinsic water permeability of polyamide material showed less variation (0.53 × 10³ to 1.56 × 10³ LMH bar^-1 nm). Contrary to common belief, both the substrate and transverse transport contributed significant resistances of (2.4 ± 1.3) × 10¹² and 5 × 10¹² m^-1, respectively. These two resistances became particularly non-negligible for membranes with thinner polyamide films. Our study provides the first detailed quantitative analysis of key contributors to hydraulic resistance and provides valuable insights for high-permeable NF membranes.